Add modern UI enhancements over the classic redesign

Layered progressive polish on the warm classic + brass theme:
- Card entrance animation on first mount (virtualizer-aware)
- Cursor-tracking brass spotlight border on cards
- Thumbnail skeleton shimmer until the poster paints
- Hover video preview after a short dwell (only when formats resolved)
- View Transition + blurred-poster ambient backdrop on player open
- Favorite heart pop + expanding ring on add
- ⌘K command palette (search, theme, density, reels, source/channel)
- Scroll-progress bar + back-to-top FAB
- Grid density toggle (comfortable/compact)
- Reels HUD: serif title, brass scrubber, muted-state pulse

All new motion respects prefers-reduced-motion; no JS/HTML structure
changes to the core grid/feed. New glue lives in frontend/js/enhance.js.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/js/feed.js

@@ -749,5 +749,8 @@ App.feed = App.feed || {};
             ? 'https://cdn.jsdelivr.net/npm/heroicons@2.0.13/24/outline/speaker-x-mark.svg'
             : 'https://cdn.jsdelivr.net/npm/heroicons@2.0.13/24/outline/speaker-wave.svg';
         icon.alt = state.feedMuted ? 'Unmute' : 'Mute';
+        // Pulse a brass ring while muted to hint "tap to hear sound".
+        const btn = document.getElementById('feed-mute-btn');
+        if (btn) btn.classList.toggle('is-muted', !!state.feedMuted);
     };
 })();